GCC 7 rejects the usage of --target=mipsel-linux-gnu, which earlier versions
accepted but warned on. Instead, detect the compiler being used and supply
--target=mipsel-linux-gnu for Clang only.
Additionally, -mips32r2 requires -mabi=32 for versions of GCC which target
mips64 by default.
This resolves PR37002.
Thanks to James Cowgill for reporting the issue!